As a renewable energy resource, the use of solar cell modules is rapidly expanding. With increasingly complex solar cell modules, also referred to as photovoltaic modules, comes an increased demand for enhanced functional encapsulant materials. Photovoltaic (solar) cell modules are units that convert light energy into electrical energy. Typical or conventional construction of a solar cell module consists of at least 5 structural layers. The layers of a conventional solar cell module are constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by light) and continuing to the backing (the layer furthest removed from the incident layer): (1) incident layer or front-sheet, (2) front-sheet (or first) encapsulant layer, (3) voltage-generating layer (or solar cell layer), (4) back-sheet (second) encapsulant layer, and (5) backing layer or back-sheet. The function of the incident layer is to provide a transparent protective window that will allow sunlight into the solar cell module. The incident layer is typically a glass plate or a thin polymeric film, such as a fluoropolymer film, but could conceivably be any material that is transparent to sunlight.
The encapsulant layers of solar cell modules are designed to encapsulate and protect the fragile voltage-generating layer. Generally, a solar cell module will incorporate at least two encapsulant layers sandwiched around the voltage-generating layer. The optical properties of the front-sheet encapsulant layer must be such that light can be effectively transmitted to the voltage-generating layer. Over the years, a wide variety of polymeric interlayers have been developed to be used as encapsulant layers. In general, these polymeric interlayers must possess a combination of characteristics including very high optical clarity, low haze, high impact resistance, shock absorbance, excellent ultraviolet (UV) light resistance, good long term thermal stability, excellent adhesion to glass and other solar cell laminate layers, low UV light transmittance, low moisture absorption, high moisture resistance, excellent long term weatherability, among other requirements. Encapsulant materials that are either currently utilized in the field or suggested to be useful include complex, multicomponent compositions based on ethylene vinyl acetate (EVA), ionomer, polyvinylbutyral (PVB), thermoplastic polyurethane (TPU), polyvinylchloride (PVC), metallocene-catalyzed linear low density polyethylenes, polyolefin block elastomers, ethylene acrylate ester copolymers, such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate), acid copolymers, silicone elastomers, epoxy resins, and the like.
EVA compositions, which have commonly been utilized as the encapsulant layer within solar cell modules, suffer the shortcomings of low adhesion to the other components incorporated within the solar cell module, low creep resistance during the lamination process and end-use and low weathering and light stability. These shortcomings have generally been overcome through the formulation of adhesion primers, peroxide curing agents, and thermal and UV stabilizer packages into the EVA compositions, which complicates the sheet production and ensuing lamination processes.
A more recent development has been the use of higher modulus ethylene copolymer ionomers within solar cell construction. For example, Maruyama, et. al. in Japanese Patent No. JP S56-116047, have disclosed a bi-layer encapsulant layer wherein the first layer is derived from EVA with a low VA content (20 wt % or lower), polyethylene, or soft PVC and the second layer derived from ionomer or EVA with high VA content (20 wt % or higher). Also exemplified is an ionomer/EVA bi-layer for use as an encapsulant layer, with the ionomer surface in contact with the outer solar cell layers, such as glass.
Okaniwa, et. al., in Japanese Patent No. JP H2-94574(A), have disclosed embossed films, such as PET film, to diffuse the incident light going into the solar cell. They further disclose “a tacky adhesive film layer” encapsulant, such as EVA, PVB, ionomer and polyethylene resins, with an encapsulant thickness of between 20 and 300 μm (0.8-11 mils).
Baum, Bernard, et. al., in “Solar Collectors. Final Report”, DOE/CS/35359-T1 (DE84011480), DOE6081.1, Contract No. AC4-78CS35359 (Springborn Laboratories, Inc.), June, 1983, have disclosed a list of usable solar cell encapsulant materials which include ethylene acrylic acid, EAA-435® (a product of Dow Chemical Co.) and ionomer, Surlyn® 1707 (a product of E. I. du Pont de Nemours and Company).
U.S. Pat. Nos. 5,476,553; 5,478,402; and 5,733,382 have disclosed the use of encapsulant layers derived from ionomers formed by the partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with inorganic bases and the use of Surlyn® 1601 and Surlyn® 1707 (E. I. du Pont de Nemours and Company).
U.S. Pat. Nos. 5,741,370; 5,762,720 and 5,986,203 have disclosed a solar cell module back-sheet which is a thermoplastic polyolefin including a mixture of at least two ionomers.
U.S. Pat. Nos. 6,114,046 and 6,353,042, have disclosed a solar cell laminate encapsulant material which includes a layer of metallocene polyethylene disposed between two layers of ionomer.
U.S. Pat. No. 6,319,596 has disclosed a solar cell encapsulant layer comprising a polyolefin with an ionomer surface.
U.S. Pat. Nos. 6,320,116 and 6,586,271 have disclosed solar cell encapsulant layers with reduced creep through treatment with electron beam radiation.
U.S. Pat. No. 6,690,930 and US Patent Application No. 2003/0000568, have disclosed the use of Surlyn® 1705-1 and Surlyn® 1706 (E. I. du Pont de Nemours and Company) zinc ionomers as solar cell encapsulants.
However, none of the currently known encapsulant layer materials can encompass all of the end-use requirements. Ionomer compositions have excellent weatherability and adhesion to other solar cell laminate layers, such as glass, but tend to be high modulus with reduced shock absorbance imparted to the solar cell.
There is a continuing need to provide solar cell encapsulant layers which provide adequate protection to the solar cell, have a long lifetime within the end-use and provide adequate adhesion to the other solar cell laminate layers, preferably without the use of adhesion primers to simplify the production processes. The use of certain ethylene acrylate ester copolymers as solar cell encapsulants has been considered since essentially the inception of solar cell modules. For example, U.S. Pat. No. 3,957,537, has disclosed certain solar cell adhesives which include ethylene copolymers. U.S. Pat. Nos. 5,582,653; 5,728,230; 6,075,202; 6,232,544; and 6,940,008, European Patent Nos. EP 0 755 080 and EP 1 544 921, and US Patent Application No. 2004/0191422 have disclosed ethylene/methyl acrylate (EMA) and ethylene/ethyl acrylate (EEA) as filler materials for photovoltaic devices.
However, it has generally been found that ethylene acrylate esters do not provide adhesion to commonly utilized solar cell structural layers, such as glass. Ethylene acrylate copolymers are also not typically optically clear. For the very same reason, U.S. Pat. Nos. 6,414,236 and 6,693,237 teach against the use of ethylene acrylate ester copolymers as solar cell encapsulants. This shortcoming has usually been overcome through the addition of adhesion primers. In addition, the ethylene acrylate ester materials have been found to creep under end-use conditions, such as in roof solar cell modules which can reach high temperatures. To overcome this shortcoming, the ethylene acrylate esters have been cross-linked, typically through the incorporation of organic peroxides with concurrent curing processes thereafter. For example, Willis, Paul B., in “Investigation Of Materials And Processes For Solar Cell Encapsulation”, DOE/JPL-954527-86/29, JPL Contract 954527, S/L Project 6072.1, August, 1986, disclose solar cell encapsulants derived from peroxide cross-linked EMA, which requires the use of a primer for effective and reliable bonding to the other components.
Tailored multilayer solar cell encapsulant layers have been developed to provide each layer's best attributes, while reducing their shortcomings. However, care must be taken when designing multilayer solar cell encapsulant layers to avoid unforeseen shortcomings which detract from the overall desirability of the resultant encapsulant layer. For example, U.S. Pat. Nos. 6,114,046; 6,586,271; and 6,353,042 have disclosed a solar cell encapsulant sheet which includes a layer of metallocene polyethylene disposed between two layers of ionomer and U.S. Pat. Nos. 6,187,448 and 6,320,116 have disclosed a solar cell encapsulant sheet which includes a layer of metallocene polyethylene disposed between two layers of an acid copolymer of polyethylene. In both cases, the tri-layer encapsulant sheet suffers the shortcoming of appearing cloudy and light blue.
The present invention overcomes these shortcoming and provides tailored, low modulus multilayer solar cell encapsulant sheets which provide excellent protection to the solar cell from physical damage, excellent adhesion to the other solar cell laminate layers, and long term stability to thermal and UV degradation.